Organosilanes of the general formula RnSiX4-n where n is an integer from 0 to 3, but more generally from 0 to 2 (where when n is 3, the organosilanes may only dimerize); R is a non-hydrolyzable organic group, such as, but not limited to, alkyl, aromatic, organofunctional, or a combination thereof, and X is alkoxy, such as methoxy or ethoxy, are prone to self-condensation rendering such organosilanes unstable in water over commercially relevant periods of time. Additionally, X can be a halogen, such as Cl, Br, or I, and is similarly liberated as HCl, HBr, or HI, For such organosilanes, the X moiety reacts with various hydroxyl containing molecules in aqueous media to liberate methanol, ethanol, HCl, HBr, HI, H2O, acetic acid, or an unsubstituted or substituted carboxylic acid and to form the hydroxylated, but condensation-prone compound.
For organosilanes RnSiX4-n, where n is an integer from 0 to 2, hydrolysis of the first two X groups with water produces a species bearing —Si(OH)2— units that can self-condense through the hydroxyl moieties to linear and/or cyclic oligomers possessing the partial structure HO—Si—(O—Si)mm—O—Si—O—Si—O—Si—OH, where mm is an integer such that an oligomer is formed. For those cases, RSiX3, hydrolysis of the third X group generates a silanetriol (RSi(OH)3), which produces insoluble organosilicon polymers through linear and/or cyclic self-condensation of the Si(OH) units. This water induced self-condensation generally precludes storage of most organosilanes RnSiX4-n, where n ranges from 0 to 2, inclusive, in water. Except for some organosilanes that are stable in very dilute solutions at specific pH ranges, the use of water solutions of most organosilanes require the use of freshly prepared solutions.
One commercially relevant example of an organosilane suffering from such undesirable self-condensation is the antimicrobial Dow Corning 5700 (Dow Corning Corporation, Midland, Mich.). The literature describes the active ingredient of Dow Corning 5700 as 3-(trimethoxysilyl)propyl-dimethyloctadecyl ammonium chloride. However, in aqueous media, it is believed that the correct active ingredient is more likely 3-(trihydroxysilyl)propyl-dimethyloctadecyl ammonium chloride. Nonetheless, Dow Corning 5700 is a water activated antimicrobial integrated system that is capable of binding to a wide variety of natural and synthetic substrates, including fibers and fabrics, to produce a durable surface or fabric coating. 3-(Trimethoxysilyl)propyl-dimethyloctadecyl ammonium chloride is prepared by quaternization of dimethyloctadecylamine with 3-chloropropyl trimethoxysilane.
The C18 hydrocarbon chain quaternary ammonium portion of the molecule possesses long-acting antimicrobial properties and provides initial association with the surface of the substrate through ionic bonds and/or electrostatic interaction. Preferably, the treated surface becomes permanently coated with a covalently bound octadecylammonium ion providing a durable, long-acting antimicrobial coating that is able to destroy microbes that come into contact with the surface.
Unfortunately, as noted above, organosilanes in water, such as the activated mixture of 3-(trimethoxysilyl)propyl-dimethyloctadecyl ammonium chloride and water are generally unstable and prone to self-condensation. For instance, the mixture of 3-(trimethoxysilylpropyl-dimethyloctadecyl ammonium chloride and water begins to lose effectiveness in as little as four to eight hours. Gel formation in this and similar silane formulations in water begins to occur in even shorter times. The limitations of such organosilanes in aqueous media are further described in U.S. Pat. No. 5,411,585, the contents of which are hereby incorporated by this reference. Moreover, such products are notorious for agitation difficulty during the addition of the silane to water. A limited list of additives useful for stabilizing organosilanes in water are described in U.S. Pat. Nos. 5,954,069; 6,113,815; 6,469,120; and 6,762,172.
The use of quaternary ammonium silicon compounds as antimicrobial agents in accordance with the prior art is well known and taught in a wide variety of United States patents, for example, U.S. Pat. Nos. 3,560,385; 3,794,736; and 3,814,739; the contents of which are hereby incorporated by reference. It is also taught that these compounds possess certain antimicrobial properties that make them valuable and very useful for a variety of surfaces, substrates, instruments, and applications (see, for example, U.S. Pat. Nos. 3,730,701; 3,794,736; 3,860,709; 4,282,366; 4,504,541; 4,615,937; 4,692,374; 4,408,996; and 4,414,268; the contents of which are hereby incorporated by reference). While these quaternary ammonium silicon compounds have been employed to sterilize or disinfect many surfaces, their employment is still limited because of their toxicity often as a result of the solvent system used to deliver the compound, the necessity for a solvent solution (for instance, Dow Corning antimicrobial agents contain 50% methanol), short term stability (stability of aqueous silane solutions varies from hours to several weeks only), and poor water solubility. For instance, while 3-(trimethoxysilyl)propyl-dimethyloctadecyl ammonium chloride does not suffer from water insolubility, it is difficult to dissolve in water and tends to form lumps before it slowly dissolves. It is unstable in water, and, because it is shipped in 50% methanol, it is overly toxic and flammable. Many other antimicrobial organosilanes, especially quaternary ammonium silicon compounds, also suffer from problems associated with physical health hazards—precautions must be taken to avoid contact with both skin and eyes, accidental spills to the surrounding area, flammability, and the added manufacturing steps needed in order to incorporate such antimicrobial agents into other articles and surfaces, resulting in much higher manufacturing costs.
Therefore, there exists a need for extended shelf-life, water-stable organosilane compounds, products, and compositions whereby, upon application, the active portion of the organosilane is operative for the selected application. Moreover, there exists a need for water-stable, organosilane compounds, products, and compositions that are essentially non-toxic, non-flammable, uniformly dispersible, and simple and economical to use. There also exists a need for highly concentrated organosilane compositions that are essentially non-toxic or of low toxicity, non-flammable, uniformly dispersible, simple and economical to use, and stable in water when further diluted with water.